1. Field of the Invention
The present invention relates to an ink jet printing apparatus for performing a printing operation by ejecting liquid such as ink or the like from ejecting ports in the form of ejected liquid droplets and allowing the liquid droplets to adhere to a printing medium such as paper, cloth, unwoven cloth, OHP paper or the like. More particularly, the present invention relates to an ink jet printing apparatus for continuously performing a printing operation for a printing medium having a large width for a long time like an industrial textile printing apparatus.
As concrete applicable instruments and apparatuses of the present invention, a textile printing apparatus, a large-sized printer and so forth can be noted.
The term "printing" mentioned in this specification with reference to the accompanying drawings is used not only for representing an operation for imparting ink to a printing medium such as paper or the like but also for widely representing an operation for imparting to the printing medium adequate liquid containing pigment and dyestuff.
2. Description of the Related Art
From the viewpoint that a high speed printing operation can quietly be performed at a low running cost, many ink jet printing apparatuses are used as a printing section for a printer, facsimile, copying machine or the like, and in accordance with the ink jet printing process, a printing operation is performed by ejecting ink from a plurality of ink ejecting ports in response to a printing data signal and then causing liquid droplets to adhere to a printing medium.
In general, the viscosity of ink used for the ink jet printing apparatus varies depending on the temperature. As the ink viscosity varies, a quantity of ink ejection at the time of each printing operation varies, causing the diameter of ink dot adhering to the printing medium to vary. Since the variation of the dot diameter is not recognized by human being's eyes as long as the variation of the ink viscosity is kept small, there does not arise a practical problem. However, in the case that the ink viscosity varies largely, this is recognized as variation of density, resulting in a problem that desired printing density can not be obtained.
In the case that the dot diameter varies depending on the place on a same printing medium, there arises a problem that this is recognized as so-called density fluctuation. For this reason, it is desirable that the temperature of ink is controlled within the range of certain extent (the range that the variation of density can not be recognized by human being's eyes) from the viewpoint that both requirements for stabilization of printing quality and cost reduction by simple controlling are satisfactorily met.
A method of controlling the temperature of ink is actually practiced in the form of temperature controlling for the printing heads. With this method, the variation of the environment temperature in the proximity of the ink jet printing apparatus is considered as a factor for inducing the temperature variation of printing heads, but generally, since the ink jet printing apparatus is often used at a room temperature (ranging from about 20 to 25.degree. C.), the environment temperature is kept comparatively stable. Rather, many factors for inducing the temperature variation of the printing heads consist in elevation of the temperature caused by heat generation at the time of driving of the printing heads. For example, in the case of a serial printer, since a quantity of heat accumulated in the printing heads is different at the time of starting of a printing operation as well as at the time of completion of the printing operation during single scanning, there arises an occasion that the dot diameter is different at the time around the starting of the printing operation as well as at the time around the completion of the printing operation.
Generally, in a printer for a printing medium having a comparatively narrow width like a printer for an A-4 sized width or a printer adapted to operate at a comparatively slow printing speed, fluctuation of the dot diameter between the time around the starting of the printing operation and the time around the completion of the printing operation can be suppressed to an extent that the foregoing fluctuation can not visually be recognized by human being's eyes, merely by disposing a heater and a temperature detecting sensor in the printing head so as to optimize the temperature of the printer head by controlling the driving of the heater in response to a signal transmitted from the temperature detecting sensor.
However, in a printer having a wide printing width or a printer adapted to operate at a high printing speed, the fluctuation of the dot diameter between the time around the starting of the printing operation and the time around the completion of the printing operation is additionally enlarged. Thus, when the controlling method as mentioned above is practiced, the fluctuation of the dot diameter is visually recognized as density fluctuation by human being's eyes, and this leads to a problem that quality of each printing operation is degraded.
As a measure for solving the foregoing problem, there is existent a method of controlling a printing head within a predetermined adequate temperature range by allowing specific cooling liquid such as water or the like to come in contact with the printing head. This method is such that the specific cooling liquid such as water or the like is sucked, pressurized and discharged by driving a pump so that the printing head is cooled by causing the cooling liquid to come in contact with the printing head via a liquid passage such as a tube or the like.
However, in the case that such pressurized liquid is used, when leakage of the liquid from the liquid passage to the atmosphere occurs due to deterioration of the tube as time elapses or for an unexpected reason, there appears a fear that the liquid leaks from the liquid passage to the outside, causing the liquid to adhere to the printing medium. When the liquid adheres to the printing medium before printing operation or after printing operation, printed images are contaminated with the leaked liquid in either case.
In general, when liquid is sucked, pressurized and discharged by driving a pump, the temperature of the liquid is elevated because of generation of heat from the pump. On the assumption that a quantity of elevation of the temperature is represented by .delta.tp, when the liquid of which temperature is controlled to temperature T by temperature controlling means is sucked by the pump and discharged from the same, the temperature of the liquid discharged from the pump becomes T+.delta.tp.
In the case that the temperature of an ink jet head is controlled by feeding the liquid accumulated in a liquid accumulating section to a liquid passage of the ink jet head while the temperature of the liquid is controlled to the temperature T by the temperature controlling means, when the liquid of which temperature is controlled to the temperature T is discharged from the pump and fed to the ink jet head as it is sucked by driving the pump, the temperature of the liquid fed to the ink jet head becomes T+.delta.tp, and the temperature of the ink jet head can not be controlled to the temperature T which is a desired temperature. In this case, it is considered that the temperature of the ink jet head may be controlled to the temperature T+.delta.tp. In general, however, values of temperature T and a quantity .delta.tp of elevation of temperature are not constant but they fluctuate. For this reason, when the liquid of which temperature is controlled to the temperature T is fed to the ink jet head while it is sucked by the pump and discharged from the same, fluctuation of the temperature is enlarged by the quantity .delta.tp of elevation of temperature achieved by the pump.
On the other hand, in the case that a feeding tube for the temperature controlling liquid in the printing head is arranged while it comes in direct contact with the printing head, since the feeding tube is displaced in the scanning direction as the printing head is reciprocably displaced in the scanning direction at the time of printing operation, the feeding tube is frequently bent and vibration caused by the frequent bending of the feeding tube is transmitted to the printing head. Consequently, the scanning speed of the printing head becomes unstable, causing a malfunction such as ink density fluctuation or the like to occur at the time of printing operation.
In addition, when a negative pressure pump is used for the purpose of recirculation of the liquid, it is necessary that the feeding tube has sufficient rigidity so as to assure that the feeding tube does not collapse under the influence of negative pressure generated by suction of the negative pressure pump. Thus, vibration caused by bending of the feeding tube is additionally enlarged with the result that the quality of printed images is largely adversely affected.